Cellulosic pulp based products comprise one of the largest and most important commodity articles of commerce. The technology for cellulosic pulp based paper products is well developed for virgin pulp and many additives are employed to improve various properties. These properties include wet and dry tensile strength, wet and dry tear strength, wet and dry burst strength, oil resistance, fold resistance, stain resistance, printability and the like. The use of recycled paper based materials is a large emerging application area of considerable importance to the pulp and paper industry. The utility of recycled pulp based products include both 100% recycled products as well as in admixture with virgin pulp.
Both areas will generally require additives to upgrade the physical properties of the resultant paper, cardboard or tissue (e.g. dry strength, wet strength, burst strength, tear resistance) as the recycled paper will show diminished properties in these categories. In some cases, the loss in physical properties could be counterbalanced by utility of more recycled pulp (higher basis weight). However, the added weight is not desired in cases where light weight products are required or where increased shipping weight would not be economically acceptable. In many cases, the standards for container packaging require certain physical property conditions which have been established for virgin pulp based products and cannot be readily achieved with 100% recycle or recycle/virgin combinations. Polymeric additives (e.g. cationic starch, cationic poly acrylamides) are often added to improve dry strength, however, offer only limited applicability to wet strength.
Wet strength additives (e.g. poly(amido amine/epichlorohydrin) require a thermal treatment or long aging at ambient conditions to generate wet strength via crosslinking reactions. Once crosslinking has occurred, the paper product cannot be easily repulped unless special treatments are conducted to break-up the chemical crosslinks. Another common wet strength additive is a formaldehyde based polymer (urea-formaldehyde and melamine formaldehyde resins). These polymers not only have repulpability problems but another environmental concern, namely, formaldehyde emissions. These polymers have recently fallen out of favor because of formaldehyde emissions and are being rapidly replaced. An additive offering both wet and dry strength improvements combined with the ability to be recycled is desired in pulp/paper recycled materials to bring performance standards up to that typically observed with virgin pulp. In essence, an additive which improves the properties of recycled paper but which renders the product non-recyclable is not desired as many recycled paper products may be also recycled.
Additives to paper to improve the property balance are characterized by the point of addition in the paper-making process. Generally, the point of addition is referenced to as wet-end or dry-end addition. Wet-end addition involves addition of the additive(s) to the pulp slurry prior to paper (or cardboard or tissue) production. Dry-end addition refers to addition of the additive(s) to the paper (cardboard or tissue) after paper formation and drying via saturation sizing or surface sizing techniques well known in the industry. The paper making process (both wet-end and dry-end addition) has been discussed in many detailed publications. One such reference is "Pulp and Paper Manufacture", Vol.7, Paper Machine Operations, B. A. Thorp, ed., TAPPI, Atlanta, 1991 (3rd edition).
Recycled paper products are significantly different than virgin pulp. One major difference is the additives which are prevalent in the recycled products are obviously not present in virgin pulp. The additives to virgin pulp can be controlled, however, this is not possible with recycled paper. These additives include not only the wet and dry strength additives noted supra but also retention aids to retain fines and fillers, alum, drainage aids, pitch or stickies control additives, fillers, inks, defoamers, and the like. The presence of these additives can result in major differences in the efficiency or applicability of additional additives to recycled paper. The problems of recycled paper is of major concern to the paper industry and has been noted in countless industry studies and publications, several of which will be cited herein. Siewert (TAPPI J., p.41, Jan. 1989) discusses the use of wastepaper in tissue production and discusses the problems with contaminants (lacquers, surface print, colored paper, and stickies). Uutela and Black (TAPPI J., p.71, Jan. 1990) reviews the use and expected future use of recycled paper and discusses some of the general problems associated with recycled paper. The problems with ink contamination was addressed by Carr (TAPPI J., p.127, Feb. 1991). The repulpability of coated corrugated cardboard was discussed by Michelman and Capella (TAPPI J., p.79, Oct. 1991). They noted that waxes, pigments, hot-melt coatings used to impart specific properties to the cardboard can affect the repulpability. These "contaminants" could also influence the properties of the recycled cardboard. Epsy and Geist (TAPPI J. p.192, July 1992) note that polyamide-epichlorohydrin resins with azetidinium or quaternary ammonium epoxide functional groups can be used to produce wet strength paper from recycled pulp. They note that dry strength improvement of recycled pulps can be achieved with additives, however, little attention had been placed on the wet strength of recycled pulps.
Polyamide-epichlorohydrin resins, however, are well known to improve wet strength at the expense of repulpability. Additives which improve wet and dry strength without sacrificing repulpability, however, are desired. Hipple (TAPPI J., p.79, May 1991) notes that secondary fiber has different characteristics than the virgin pulp on which it was based. The deinking process introduces major changes in the fiber characteristics requiring differences in the type of polymeric additives utilized in the wet-end operation. It was noted that with cationic wet-end starch the level of cationic substitution may need to be adjusted to yield the desired properties. These papers note the problems of recycled paper, the problems of contamination, and the differences encountered versus virgin pulp. The high level of anionic "trash" in newsprint, as an example, questions the utility of cationic polymers in achieving the desired property improvements due to polyelectrolyte complexation.
U.S. Pat. Nos. 4,880,497 and 4,978,427 discuss the use of amine functional polymers for use in improving the dry and wet strength of paper. These amine functional polymers are based on copolymers comprising 10 to 95 mole % N-vinyl formamide which are hydrolyzed to yield amine functionality. The copolymers also contain an ethylenically unsaturated monomer including vinyl esters (such as vinyl acetate), alkyl vinyl ethers, N-vinyl pyrrolidone, and the esters, nitriles, and amides of acrylic acid or methacrylic acid. Thus, amine functional poly(vinyl alcohols) are included as they would result from the hydrolysis of vinyl acetate/N-vinyl formamide copolymers. The problems of copolymerization to yield uniform copolymers of vinyl acetate/N-vinyl formamide above 10 mole % NVF are not noted and, indeed, the examples shown in these patents clearly do not represent random copolymers but most probably polymer mixtures of various compositions between poly(vinyl acetate) and poly(N-vinyl formamide) (before hydrolysis). Although these patents note the use in waste paper, they do not show any specific examples or note the more demanding characterization required for recycled paper. More importantly, these patents do not demonstrate that low levels of amine functionality (&lt; 10 mole %) in poly(vinyl alcohol) would be effective; and based on the claims and examples it would be unexpected that lower levels would be effective for virgin pulp. With the more severe problems with recycled paper; utility of amine functionality of less than 10 mole % would even be less expected.
European Patent Application 0 251 182 discusses a vinyl amine copolymer derived from hydrolyzed co- or terpolymers of N-vinyl formamide, (meth)acrylonitrile, (meth)acrylamide, and acrylic acid. Utility as a drainage aid in papermaking and as a paper strength additive was noted. European Patent Application 331,047 notes the utility of a high molecular weight poly(vinyl amine) as a wet-end additive in paper making for improved dry strength and fiber retention aid. European Patent Application 337,310 discusses vinyl alcohol-vinylamine copolymers in combination with an anionic polymer which increases the moist compressive strength of paper and paperboard. The specific utility of improving the wet and dry strength of recycled paper was not investigated or noted.
U.S. Pat. No. 4,772,359 notes the utility of high molecular weight water soluble poly(N-vinylamides) as drainage aids, retention aids and flocculants for paper, board and cardboard applications. U.S. Pat. No. 3,597,314 discusses a method of improving the drainage of cellulose fiber suspension with polymers of N-vinyl-N-methyl formamide. U.S. Pat. No. 3,715,336 discusses vinyl alcohol/vinylamine copolymers prepared by the hydrolysis of vinyl acetate/vinyl carbamate copolymers for utility as flocculants for aqueous suspension of inorganic solids. U.S. Pat. No. 4,311,805 discusses vinyl alcohol copolymers containing specific cationic groups for use as paper strength addition.
U.S. Pat. No. 4,421,602 notes the utility of vinylamine/N-vinyl formamide copolymers for use as retention agents, drainage aids and flocculants for use in papermaking. U.S. Pat. No. 4,614,762 discusses a water soluble product of polyethyleneimine reacted with formaldehyde and poly (vinyl alcohol). The product is noted to be useful as an improved drainage and retention aid in papermaking. U.S. Pat. 4,808,683 discloses vinylamine copolymers which offer utility as flocculating agents, drainage aids, and paper strength additives. U.S. Pat. No. 3,535,288 discusses cationic poly(amide-epichlorohydrin) thermosetting resins for use in the production of paper having improved wet strength.
A comprehensive review of polymeric additives incorporated in paper manufacture is given by G. G. Spence in Encyclopedia of Polymer Science and Engineering, 2nd Ed., Wiley-Interscience, Vol. 10, p. 761-786, New York, 1987. This review discusses in detail the various additives commonly incorporated in virgin pulp for various paper based products, however, does not review the relevance of these additives for recycled paper.